The present invention relates to a thin ribbon of a rare earth/iron/boron-based magnet alloy prepared by the strip casting method and useful as a base material of a rare earth-based permanent magnet having greatly improved magnetic properties.
Permanent magnets belong to a class of very important key materials in a great variety of instruments built under the most advanced technology in the field of electric and electronic industries covering from household electric appliances in general to peripheral terminals of computers and medical instruments. Along with the recent progress in the fields of computers and communication instruments, the electric and electronic instruments are desired to be more and more compact in size and lighter and lighter in weight and to exhibit higher and higher performance. It is apparent that these requirements cannot be satisfied without great upgrading of the permanent magnets built in the instruments as a key component.
As is well known, rare earth-based permanent magnets are widely used in the above mentioned fields as a class of high-performance permanent magnets, of which the rare earth/iron/boron-based magnets are the most promising in respect of their outstandingly excellent magnetic properties and economical advantages due to the relatively low material costs. The rare earth/iron/boron-based or, in particular, neodymium/iron/boron-based permanent magnet alloy is prepared usually by the mold casting method or by the strip casting method. The magnet alloy prepared by these methods is processed into a permanent magnet by the well known powder metallurgical process involving the step of magnetic orientation of the alloy particles under compression molding in a magnetic field.
In the above mentioned mold casting method for the preparation of a magnet alloy, the constituent elements each in the metallic or elementary form are melted together in a crucible to give an alloy melt which is cast into a casting mold followed by solidification therein to give an ingot of the alloy which is processed into magnets by the powder metallurgical method. This method is widely practiced in respect of the advantage that the chemical composition of the magnet alloy can easily be controlled. A problem in the mold casting method described above, however, is that, since the velocity of heat transfer is relatively low between the mold walls and the alloy melt and within the alloy melt per se, a long time is taken for solidification of the whole volume of the melt to give a solid of the magnet alloy so that the .gamma.-iron is crystallized as the primary crystals during the process of solidification of the molten alloy to form grains of the .gamma.-iron phase having a diameter of 10 .mu.m or larger left in the core portion of the ingot block. In addition, the phase rich in the content of the rare earth element, referred to as a R-rich phase hereinafter, and the R.sub.x T.sub.4 B.sub.4 phase, in which R is a rare earth element or a combination of rare earth elements, T is a transition metal element or typically iron and the subscript x is a positive number larger than 1 varying with the content of the rare earth element, surrounding the R.sub.2 T.sub.14 B phase as the principal phase in the permanent magnet are in the form of coarse grains of a large diameter.
Moreover, the metallographic structure of the alloy ingot cannot be uniform enough between the surface layer of the ingot solidified in contact with or in the vicinity of the mold wall and the core portion of the ingot remote from the surface layer due to non-uniformity in the cooling rate resulting in variations in the grain diameter of the R.sub.2 T.sub.14 B phase and the R-rich phase. Consequently, difficulties are encountered in the process of pulverization of the alloy ingot into a fine powder having a particle diameter of a few .mu.m and the particle size distribution of the alloy powder cannot be uniform enough adversely affecting the magnetic properties of the permanent magnets finally obtained by the powder metallurgical method due to poor magnetic orientation of the alloy particles and poor sintering behavior of the powder compact.
In the strip casting method, on the other hand, a melt of the magnet alloy is continuously ejected at the surface of a rotating quenching roller of the single-roller type or twin-roller type to prepare a thin ribbon of the solidified alloy having a thickness of 0.01 to 5 mm. This method is advantageous for obtaining a high-performance R/T/B-based permanent magnet because the metallographic phase structure of the magnet alloy in the form of a thin ribbon can be controlled by adequately selecting the quenching conditions of the alloy melt. For example, precipitation of the .alpha.-iron phase can be decreased or the R-rich phase and the R.sub.x T.sub.4 B.sub.4 phase can be dispersed with increased fineness and uniformity.
With an object to further improve the magnetic properties of the R/T/B-based permanent magnets obtained from a magnet alloy prepared by the strip casting method, detailed and extensive investigations have been undertaken on the metallographic structure of the thin alloy ribbon prepared by the strip casting method or, in particular, on the mode of precipitation of the .alpha.-iron phase and structure thereof leading to a proposal for a thin alloy ribbon in which the .alpha.-iron phase is finely dispersed in a size of smaller than 10 .mu.m within the crystalline grains of the principal phase as the peritectic nuclei (Japanese Patent No. 2639609) and a thin alloy ribbon substantially free from segregation of the .alpha.-iron phase (Japanese Patent No. 2665590 and Japanese Patent Kokai 7-176414).
In addition to the above, a great number of reports, in compliance with the unlimitedly growing desire for upgrading of rare earth-based permanent magnets, are dedicated to the method for the preparation of the R/T/B-based permanent magnets. Despite the so large number of reports in this field, almost no reports are available on the relationship between the phase-precipitation mode or metallographic phase structure of the thin alloy ribbons and the magnetic properties of the permanent magnets obtained therefrom by directing attention to the region where four phases are jointly found, referred to as the four-phase region hereinafter, including, in addition to the .alpha.-iron phase, the R-rich phase and the R.sub.x T.sub.4 B.sub.4 phase in combination with the R.sub.2 T.sub.14 B phase as the principal phase.